She started for the team in major international competition for five years and helped the Americans win Olympic gold in 2008. Astute observers of the sport consider her the most complete and versatile player in the esteemed program. She was active all summer, featuring in the midfield for the Atlanta Beat in Women's Professional Soccer.

But Chalupny, 26, will not play for the U.S. squad in Mexico and perhaps never represent her country again because of a history of concussions. Although Chalupny has been allowed to continue her pro career, the U.S. Soccer Federation, in consultation with one of the leading experts on sports head trauma, has not cleared her to play for the national team.

"I am 100 percent healthy and I have doctors to back me up," Chalupny said this week. "If I felt I was putting myself at risk, I wouldn't be playing."

The USSF, the sport's governing body in the United States that oversees all national teams, declined to elaborate on the decision, citing privacy issues involving an individual's medical records. In head injury cases, the USSF turns to Ruben Echemendia, a clinical neuropsychologist who has worked with the soccer organization for eight years and oversees the NHL and MLS concussion programs.

Chalupny's situation comes at a time of heightened concern about concussions suffered in athletic competition. After a series of head injuries this season, the NFL has cracked down on dangerous hits by implementing more severe disciplinary measures.

In soccer, MLS veterans Taylor Twellman, Alecko Eskandarian and Bryan Namoff have been sidelined indefinitely because of concussions. Washington Freedom goalkeeper Briana Scurry was out for almost all of the WPS season with a head injury before retiring last month.

Athletes in many sports now must undergo an evaluation period before being allowed to return, a cautious approach spurred by medical advances and evidence of long-term damage to those who have endured additional blows to the head.

Chalupny's case is unique in that she is allowed to play for her pro team but banned from playing for her national team.

In an interview, Echemendia, who is in private practice after 18 years on the Penn State faculty, said he couldn't discuss Chalupny's case. But speaking in general about evaluating head injuries, he said he looks at the number of concussions that an athlete has suffered, how closely over time they occurred, the severity of symptoms and how long those symptoms lasted.

Chalupny, who also played for the University of North Carolina and U.S. junior national teams, said doctors have diagnosed her with a concussion five or six times.

While with the U.S. senior team in 2006, she suffered one against France, sidelining her for four months. Two years later, in the opening match at the Olympics, U.S. goalkeeper Hope Solo inadvertently punched her in the head on a clearing attempt. Chalupny left that game, missed the next match and returned to play in the last four.